This also adds ImageRegistry, a SVG cache, a bunch of utility functions
for shared pixmaps in CalamaresUtilsGui and renames several qrc files
in viewmodules.

Now linking with -Wl,--no-undefined!
Moved find_package KF5 from the root CMakeLists.txt to the partitioning
module where it belongs. KF5 deps should be limited to plugins.
Lots of random fixage to make stuff link.
Removed CalamaresApplication reference from Settings.
Removed the use of AbstractPage, we don't need it right now.

The qt_* macros have a different signature

Added ViewManager.
Added dummy Settings class.
Added dummy plugin interface (UI plugins only).
Added dummy greeting plugin.
Added DLLEXPORT macros for UI plugin interface and plugins.